Upon recovery, cores from Hole 990A were cut into 1.5-m sections and split longitudinally into archive- and working-half cores. P-wave velocities were measured at 2-5 cm intervals at bench pressure, and first arrivals were picked by eye as soon as possible after core recovery to ensure seawater saturation (Figure 1).

The P-wave velocities shown in Figure 1 vary from ~2.2-6.5 km s^-1. This relatively large range of P-wave velocities reflects changes in rock properties (e.g., mineralogy and pore shape distribution) down the core. Cyclic velocity patterns are evident throughout the profile. Each full cycle (i.e., low velocity-high velocity-low velocity) corresponds to a single lava flow unit. Shipboard petrologists defined flow boundaries based on visual and microscopic observations of the recovered cores. The petrologically defined flow units correspond to the full cycles observed in the shipboard P-wave velocity measurements (Fig. 1). The low velocity flow tops and bottoms are characterized by large amounts of alteration products (e.g., clay minerals) and high total porosity. The higher velocity sections consist of massive basalt within the centers of the flow units. Fresh basalt is more prevalent and total porosity is lower in these central sections than in the flow tops and bottoms (Shipboard Scientific Party, 1996; Planke et al., Chap. 2, this volume).

Shipboard petrologists have described the upper three flow units as aa type flows (Shipboard Scientific Party, 1996). We chose to sample and analyze basalt from the upper three flow units because the distributions of vesicles from these flows are similar. The sample locations and shipboard P-wave velocity measurements made at bench pressure are shown in Figure 1.